Milk snakes still are a good size snake depending on the species tho.... Up to 6 feet. Now a ball python bred to be more compact and that skinny would just look emaciated and sad.
Also keep in mind that in some animals (dogs and cats or lion/tiger crossbreeds) you get issues with going too large as well. Heart problems or different types of bone deformities than going mini can lead to. Or shortened lifespans.
A 5 pound chihuahua? 12-20 years... vs 8-10 on a great dane
Reptiles are a weird spot tho as they technically continue growing throughout their life, just slow down greatly with age. Obviously selective breeding as mentioned before is key.
though the founding leopard gecko for the giant line died unexpectedly at only age 6. So who knows if you would end up with similar issues on snakes.